Creating and implementing effective seals for rotating equipment has been an effort for almost as long as there has been rotating equipment. And just as there is a broad array in the applications and types of rotating equipment there is also a broad array of seals that are employed in this equipment.
One of the simplest and oldest type of seals, a packing based seal, is still often employed. In this seal a gland can be tightened to compress the packing around the shaft. So there is a balance between how tight to make the packing which causes frictional losses and wears the shaft and how effective the seal is. Lip seals are in another form of contact based seals. They also can wear grooves into the shaft and are subject to wear and leakage themselves. Labyrinth seals are a form of non-contact seal but they provide a conductance path that can result in huge flows when there is significant pressure differentials across the seal. In order to minimize the leakages clearances between the rotating in stationary sections of the seal are minimized to the extent possible. This adds significant costs and still to make them effective they often need to be relatively long axially. There are brush and ablatable type seals which are contact based seals often employing centrifugal force or pressure differentials to keep them in contact with their mating surface. These seals create particulate and are a wear item that becomes a maintenance cost, at high speeds they create significant amounts of heat and frictional losses. Additionally these contact seals create a lot of noise. Bearing isolators are commonly found in process equipment, they typically combine labyrinth and lip type seal technologies and sometimes employ and injection of a fluid or gas at a pressure above that of the volume to be sealed. In example of such an art would be; U.S. Pat. No. 7,631,878 to Orlowski et al.
Mechanical seals and dry gas seals also could be considered injection type seals, as these seals often have some type of a flush or seal gas employed in them. Dry gas seals specifically use hydrodynamic air bearing affects to create very small non-contact gaps that are very effective at sealing. These sealing effects are dependent upon relatively high surface speeds between the sealing surfaces. There is a lot of engineering that goes into the seals in order to keep their bearing surfaces flat and pressed against each other so as to prevent a seizure from contact at speed between the bearing surfaces or a failure to seal because a mechanism that is used to provide axial compliance “Hangs Up” allowing there to be a large gap between the sealing surfaces. Mechanical seals also suffer from the same issues but there sealing surfaces are designed to be relatively good plain bearing partners, still because they are often contact bearings they wear and they create heat. An example of such a seal would be; U.S. Pat. No. 7,823,885 to Droscher which does a good job describing the problems with conventional seals and “hang-ups” and is also an example of a conventional injection type seal. Additionally it is noted that this injection fluid may also help to establish an aerodynamic bearing property at the face of the seal.
The state-of-the-art today includes hydrodynamic bearings such as spiral groove and foil bearings which can become noncontact based on the viscous dragging of a fluid or gas into a gap, there are gas seals and labyrinth seals that attempt to create restriction through small gaps. Examples of gas seals include Pall Corporation and Carbone Turbograph seals. In the case of Carbone they are manufacturers of porous media carbons and graphite's but they do not employ porous media or graphite as a compensation technique for hydrostatic sealing purposes. We suggest this is evidence that using such compensation techniques through porous media is not obvious to them.
In another contemporary example US patent application publication No. 2006/0062499 to Boyd specifically teaching and claiming the use of carbon graphite and ceramic materials and using pressurized gas does not employ porous media compensation. This patent is targeted specifically towards high-speed turbine engines. We suggested this as an example that the use of porous compensation is not obvious.